A maskant has been developed for use during laser welding or drilling of components, particularly components with protective coatings. The maskant is particularly useful when performing laser welding operations on turbine blades with ceramic thermal barrier coatings. The maskant improves the yield when performing weld operations on certain components, thereby reducing rework, scrap, and associated costs.
Current configuration turbine blades receive ceramic thermal barrier coatings. The ceramic coatings are typically in the range of 0.003xe2x80x3-0.00711xe2x80x3 thick. Conventional weld repair procedures require the removal of any coatings from the base alloy immediately adjacent to the weld areas to avoid contamination of the weld and to minimize cracking. During welding of the tip of a gas turbine engine blade when the laser beam makes the initial passes around the blade there is xe2x80x9coverhangingxe2x80x9d laser light that passes down the sides of the airfoil. The overhanging laser light does not cause damage on many part configurations; however, when blades exhibit a step caused by the local removal of the thermal barrier coating there is a problem. The overhanging beam often strikes the edge of the ceramic. The ceramic material (generally yittria stabilized zirconia) has a tendency to preferentially absorb the laser light from a CO2 laser. The very low thermal conductivity of the ceramic does not allow the heat to dissipate readily. The ceramic then melts, causing subsequent melting and deep pitting of the base alloy, resulting in damage or destruction of the part being welded.
One solution is to remove the ceramic to a level where it will not melt when hit by the laser beam (because the beam is sufficiently defocused). Unfortunately, removing the ceramic to that level exceeds the maximum allowable amount of coating removal for certain types of repair. The laser beam cannot be positioned in such a way that it does not overhang the edge of the blade, or there will be incomplete fusion at the base of the weld.
Another solution is to physically mask the edge of the ceramic to protect it from the laser beam. The edge of the ceramic can not be readily protected by a hard mechanical mask, since the mask must fit the part perfectly in order to block the laser beam. Custom masks would have to be created for each blade to be welded in order to take into account slight variations in the contour of each individual piece. Production of such custom masks is prohibitively time consuming and expensive.
This invention describes a method of protecting the edge of the ceramic thermal barrier coating on a component by coating the ceramic edge with a suitable maskant. The maskant material is capable of withstanding the extreme temperatures encountered during the laser weld or drilling process. The maskant is also able to dissipate the concentrated heat absorbed when struck by the laser beam. The maskant is also inert with respect to the component being welded.
The maskant of this invention consists of a mixture of powdered metal particles, silica and solvent (preferably water) in amounts sufficient to form a paste-like consistency. Preferably the paste is metallic powder in a suspension of colloidal silica in water. While a binder can be used to bind the paste to the component, preferably a colloidal silica acts as a binder to hold the metal powder together and to adhere the paste to the part being welded. The paste consistency is easy to apply to the component surface and conforms to the non-uniform shapes. The metal powder used is compatible with the base alloy of the metallic component, ie. it will not contaminate or detrimentally affect the component alloy. Preferably the metallic particles are the same alloy powder used as filler by the laser welding system and typically the powder is the same as the component base alloy.
Generally the colloidal silica in a water suspension is added to the metallic powder in a quantity sufficient to form a paste-like consistency. Typically the maskant comprises 75 to 90% by weight of metallic powder 3 to 10% by weight of colloidal silica and 5 to 15% by volume of water. The metal powder is mixed with the silica suspension to form a paste, which is then applied by painting over the area to be protected, e.g. covering the edge of the ceramic on the part to be welded. The maskant is capable of dissipating the heat resulting from an overhanging laser beam strike. If there is localized melting of the maskant it does not typically adhere strongly to the side of the blade, and can therefore be easily removed after welding is completed. Since the silica is inert and the metal powder is compatible with or matches the component alloy, there is no contamination. After the weld process is complete a light gritblast operation is used to clean the residual maskant from the part.
The benefits of this invention are as follows: the maskant may be easily mixed and applied to components with uneven contours and it retains its shape and adheres to the components even after drying; the composition of the maskant is easily tailored to match different base alloy configurations by changing the alloy powder used; the maskant is capable of withstanding extremely high preheat and weld temperatures-it has been tested at over 2000xc2x0 F.; the maskant does not interfere with the close fit of induction coils that are often required to preheat sophisticated superalloys prior to welding; the maskant is easily removed after use, with complete removal easily verified; and the silica binder used is completely inert, and does not act to contaminate the weld.